Any thoughts on what the best way to do this is? Everything I've seen puts the fondant above the bees either directly on the frames. Doesn't work in a top bar because there's obviously no space between the top bar and the roof.

Will it cause a problem if I slide it under the combs in a few lunchboxes or something similar? I can leave the lids on and drill holes in the sides of the lunch boxes to keep anything from falling out of the combs into the food.

From what I have learned the problem with fondant in containers in the bottom of hTBH is that it may become sticky and trap the bees and this doesn’t happen if it is suspended above the bars in a hive and the bees can get at it from underneath the receptacle.
What I did yesterday was as suggested on here and flattened some fondant in a plastic zip lock bag and stapled it above the zip lock, to the side of a follower board so the bees can get to it. I was a little worried about making long vertical cuts in the bag so instead made two vertical lines of puncture holes with just the point of the blade.
It only took a couple of seconds to swap follower boards as not to disturb or cool the hive. Today I had a peep from below via my hinged base board and the fondant is still contained in the bag and not leaked out but the bees don’t seem interested in it, perhaps they have enough stores.
Again another suggested method I would have used if I had the kit would be to attach some old hard comb to a follower board and spread the fondant into the empty cells.
HTH
Season’s Greetings to all peeps

Design of one of my two does have space over the top bars so I have a couple of bars with holes drilled in the bars which I place as close as I can to the centre without disturbing the bees. The other one I use a follower board as suggested above. One potential issue with both of these solutions is that in very cold weather the bees may not venture far enough away from their nest to find it leading to starvation within reach of food.

Conserving wild bees

Research suggests that bumble bee boxes have a very low success rate in actually attracting bees into them. We find that if you create an environment where first of all you can attract mice inside, such as a pile of stones, a drystone wall, paving slabs with intentionally made cavities underneath, this will increase the success rate.

Most bumble bee species need a dry space about the size a football, with a narrow entrance tunnel approximately 2cm in diameter and 20 cm long. Most species nest underground along the base of a linear feature such as a hedge or wall. Sites need to be sheltered and out of direct sunlight.